Final answer:
The plumber's charges describe an Increasing Linear Function with a y-intercept of $30 and a slope of $15, where the total charge is dependent on the hours worked.
Step-by-step explanation:
The plumber's charges of a $30 minimum plus $15 per hour of work is best described as an Increasing Linear Function. This is because the total charge increases at a constant rate with each additional hour of work, making it linear, and it increases as more hours are worked, hence 'increasing'.
The equation y = $30 + $15x represents this scenario, where x is the number of hours worked, and y is the total charge. The y-intercept is $30, which is the initial charge or the fixed minimum charge when no hours of work are done (when x = 0). The slope is $15, which represents the rate of the charge per hour of work.
The independent variable is the number of hours the plumber works (x), and the dependent variable is the total charge (y).